1. Field of the Invention
This invention relates to a non-heat-treatment type aluminum alloy having high ductility for diecasting and a method of manufacturing parts made of the alloy.
2. Background of the Invention
High-pressure diecasting is used for manufacturing an aluminum casting in various industries because of its good characteristics of molding and lower manufacturing cost than other casting method. As aluminum alloys used for the high-pressure diecasting, ADC5 and ADC6 defined in H5302 of JIS (Japanese Industrial Standard), or ADC7 defined in the former JIS, the ADC7 were, however, removed from the former JIS at its second amendment on Nov. 1, 1976 because of its infrequent use, are known. Recently in the automobile industry, molding a plurality of parts in one piece out of an aluminum alloy has been studied and developed for the purpose of reducing cast and weight by use of the high-pressure diecasting. However the one-piece molding can not be applied when the designed integral part has some portions with specific construction which make it impossible for a set of molding dies to be separated after finishing molding. It could be one of ways to solve this problem that an initial construction to make it possible for the molding dies to be separated is given first and, after finishing the separation of molding dies, a plastic deformation is given to the portions by bending operation to give the molded part the predetermined shape. It is however difficult to realize the idea mentioned above because aluminum alloy for diecasting has generally such a poor ductility that the bending operation tends to give a product fractures or cracks and to have it broken. To solve such a problem, Japanese Unexamined Patent Publication No. 3-122242 and 6-330202 disclose aluminum alloys with improved strength, ductility and toughness which are yielded by changing the ingredients and the proportion of the ingredients of the alloys. Japanese Unexamined Patent Publications Nos. 62-149839 and 7-252616 suggest other aluminum alloys with increased strength and abrasion resistance which is realized by increasing silica ingredient.
The prior art aluminum alloy which has relatively rich magnesium ingredient has higher strength and, however, tends to produce defects such as shrinkage cavity inside the cast product, and to give more fractures on the surface of the product, in other words, to get the fractures characteristics, which means the degree of fractures appearance, worse. Thus relatively rich magnesium ingredient aluminum alloy generally has rather poor casting characteristics. Therefor, the elongation of the cast product out of the rich magnesium aluminum alloy get smaller than that of alloy itself. In fact, a cast product out of ADC6 and other alloys as samples of the magnesium rich alloy meeting the aforesaid JIS show a smaller elongation rates than that of the alloy itself. Also an alloy disclosed in the publication Nos. 3-122242 and 6-330202 include relatively rich magnesium ingredient like the allay defined in JIS, so a good elongation of the molded product is not expected. As the alloys disclosed in the publications 62-149839 and 7-252616 have silica ingredient as relatively rich as the alloy ADC7 defined in JIS, it has slightly better casting characteristics and the degree of elongation of the alloy itself however reduces and becomes lower than that of the alloys defined in JIS. Thus generally there is an inverse relationship between casting characteristics and elongation. Namely trying to improve casting characteristics of an alloy by changing ingredients or proportion thereof results in reducing the elongation of the alloy itself and, on the other hand, making an elongation of an alloy itself higher causes the casting characteristic to get worse, which as one whole brings a reduction in elongation of the cast product. As a result it has been difficult to get the alloy having good casting characteristics in addition to good elongation so that the cast product made of the alloy can be plasmically deformed by bending or the like. Meanwhile it is possible to improve the elongation of the molded product by heat-treating or to male it easy to deform the product plasmically by keeping it at a high temperature, but that method does not always give sufficient results and yet usually cause a cost increase.